This invention relates to a method for processing fluids under high differential pressures with a positive displacement machine of the "single screw" type, i.e. in which a screw cooperates with one or a plurality of pinion-like gate-rotors.
This invention also relates to a positive displacement machine intended to work under high differential pressures.
U.S. Pat. No. 3 788 784 discloses single screw machines, which may be compressors, pumps, or expanders, having "floating gate-rotors", i.e. gate-rotors comprising a sealing portion made of plastics supported by a rigid metal support portion. The plastic sealing portion and the support portion are allowed to move angularly to a limited extent with respect to each other.
The instantaneous speed of the plastic sealing portion, having a very low inertia, can fluctuate as needed by the screw without incurring high forces, thus without incurring high contact pressures between the thread surfaces of the screw and the edges of the teeth of the gate-rotor. This limits wear.
This system has appeared to be essential to the life of the above mentioned machines.
It works very well provided that the discharge pressure remains within limits. For instance, such an air compressor operating with a discharge pressure up to 1 MPa or refrigeration compressor operating with a discharge pressure up to 3 MPa exhibits no wear of the gate-rotor after a significant time of operation, and this appears to be a result of the two-portions structure of the gate-rotor since, after such a time of operation, the plastic portion appears to have moved onto the support portion: the plastic is indeed polished where it has been in contact with the teeth of the support portion, and the polished area appears to be wider than the support teeth.
Nevertheless, when the differential pressure, (i.e. the pressure difference between intake and discharge) definitely exceeds the above mentioned values and especially when the liquid used to cool the machine--when used as a compressor--is not oil but e.g. water, which has poor lubricating properties, the pressure differential across the tooth is so high that it practically clamps the plastic sealing portion of the gate-rotor against the support portion thereof, and this can lead to wear of the gate-rotor.